Unresolved references mean that there are calls to a function, but the function itself isn't implemented anywhere. Are those functions actually defined in a .cpp file somewhere? If not, then perhaps you intended to make the functions abstract (pure virtual)?

Like I said, you only get undefined reference errors if there are actually references to a function. So as long as you don't call the function anywhere, there won't be an error if the function is not implemented. Perhaps that's what happened first time: You added a function call and this triggered the error.